In general, for enabling passengers to seat in the most comfortable position, seats each include a seat back to support the passenger's back and a seat cushion to support the hip and thighs.
A structure of a seat has a seat cushion frame and a seat back frame. A reclining device that allows a passenger to adjust the angle of the seat back forward/backward to fit his/her body shape is mounted at the joint of the seat cushion frame and the seat back frame.
Two reclining devices, as a pair mounted respectively at the left/right sides of the seat, are mounted as a whole part through spline engagement with both ends of a shaft.
The reclining device is classified into an electric type in which reclining is performed by a shaft rotated by an electric motor and a manual type in which reclining is performed by rotation of a shaft by a passenger operating a lever. Recently, as upscale image of vehicles is emphasized, the trend is to use the electric type.
On the other hand, engagement force of gears between components of the reclining device should be large to increase the overall strength of the reclining device and prevent malfunction. For this reason, the reclining device should have high strength and durability.
Further, when the seat back unnecessarily moves, a passenger feels uncomfortable, which not only deteriorates ride comfort, but makes noise. Therefore, it is required for the reclining device to have a structure that prevents the problems.